New assets:// stream wrapper for custom aggregate path location

The file location for Drupal's asset aggregation system is now configurable in settings.php via $settings['file_assets_path']. Asset files will be accessed via the assets:// stream wrapper.

This can be useful with modules that override the public:// stream wrapper to use alternative storage such as the s3fs module, where it wants to manage uploaded files, but not dynamically generated asset aggregates.

If the setting is not customized, then the files will continue to be written to the public files directory.

Modules that are generically dealing with the public:// stream wrapper will need to adapt their logic to also deal with the assets:// stream wrapper if the same processing needs to be applied to both.